1. Field of the Invention
The present invention relates to a collimator which is used in a radiotherapy apparatus and controls a radiation field by a plurality of collimating leaves. The present invention also relates to a method of positioning a plurality of collimating leaves of the collimator.
2. Discussion of the Background
It is important to reduce X-ray exposure to an object such as a patient when a radiotherapy apparatus or an X-ray diagnosis apparatus is used. Many types of techniques are presented for accomplishing the X-ray exposure reduction. One well-known technique is the use of a collimator. The collimator narrows a radiation field by reducing its aperture size. The aperture size is controllable by adjusting positions of aperture blades in intercrossing (X-Y) directions. The aperture blades are typically made of an X-ray non-transmission material such as lead or tungsten. In one example of the collimator, a lamp is provided at a position corresponding to an X-ray radiator. A shade resulting from the lamp light through the controlled aperture of the collimator is used for adjusting and determining an actual radiation field. In other words, a desired radiation field is obtained by controlling the aperture so that the shade is identical to the desired radiation field. This technique is disclosed, for example, in Japanese Patent Application Disclosure PH3-44768.
In many cases, however, a multileaf collimator is used for a radiotherapy apparatus. The multileaf collimator includes an arrangement of a plurality of collimating leaves formed in a mutually contiguous manner. The collimating leaves perform a function similar to that of the aperture blades. Instead of a pair of aperture blades, a pair of collimating members each of which include a plurality of collimating leaves are used in one direction for controlling the aperture so as to help form the shape of the aperture more appropriately to the shape of a tumor. If, however, the above-mentioned lamp is used to adjust a plurality of collimating leaves, it is not easy to obtain sufficient accuracy in terms of position adjustment. The position adjustment is usually performed at a position of the isocenter within a light emission field of the lamp. The required accuracy at the isocenter is, for example, one millimeter error. Meanwhile, a position of the collimating leaf is typically detected to the precision of 0.1 millimeter. Therefore, the detection error (i.e., incorrect detection result) of the collimating leaf position may lead to an error expansion at the isocenter. In other words, a slight detection error may result in an error beyond the required accuracy at the isocenter. Consequently, a high accuracy in the position detection is required for the collimating leaves.